1. Field of the Invention
The invention relates to microwave, millimeter wave and submillimeter wave spectroscopy systems and components and in particular to a method and apparatus for controlling the phase shift of the optical signal in a homodyne or heterodyne transceiver useful for terahertz spectroscopy.
2. Description of the Related Art
Terahertz devices and systems generally employ electromagnetic energy between 300 GHz and 3 terahertz (3 THz), or wavelengths from 100 to 1000 microns (0.1 to 1.0 millimeters), which is also referred to as the submillimeter or far-infrared region of the electromagnetic spectrum. Terahertz energy can be created, for example, using short-pulsed lasers, heterodyne lasers, electronic diode multipliers, free-electron lasers, and BWOs.
One important application of terahertz systems is THz spectroscopy. Terahertz spectroscopy presents many new instrumentation and measurement applications since certain compounds and objects can be identified and characterized by a frequency-dependent absorption, dispersion, and/or reflection of terahertz signals which pass through or are reflected from the compound or object. One implementation of such spectroscopy is time domain spectroscopy, in which a sequence of femtosecond pulses from a mode locked laser are focused onto suitable semiconductor material to produce THz radiation. The radiation is focused or directed to a target or sample to be analyzed, and a detector or detector array is used to collect any signals propagated through or reflected from the object. Since such measurements are made in the time domain by collecting the timed sequence of pulses, the signals must then be processed by a Fourier transformation to recover the desired frequency domain spectral information.
By scanning every point or “pixel” on that object, either on a focal plane or in successive focal planes at different ranges, it is also possible for such a time domain system to perform imaging of the surface or interior cross-sections or layers of the object. This non-invasive imaging technique is capable of differentiating between different materials, chemical compositions, or molecules in the interior of an object. However, the process of performing a Fourier transform from the time domain into the frequency domain imposes limitations on the frequency resolution and upon the ability to look at specific frequency windows.
As noted in a review article by Peter H. Siegel in IEEE Transactions on Microwave Theory and Techniques, Vol. 50, No. 3, 915-917 (March 2002), terahertz time-domain spectroscopy was described by Nuss and others at Bell Laboratories in the mid-1990s (B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett., vol. 20, no. 16, pp. 1716-1718, Aug. 15, 1995; D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, “T-ray imaging,” IEEE J. Select. Topics Quantum Electron., vol. 2, pp. 679-692, September 1996), and recently commercialized by at least two companies, Picometrix, LLC of Ann Arbor, Mich. (D. D. Arnone et al., “Applications of terahertz (THz) technology to medical imaging,” in Proc. SPIE Terahertz Spectroscopy Applicat. II, vol. 3823, Munich, Germany, 1999, pp. 209-219) and Teraview Ltd. (a spinoff of Toshiba Research Europe) located in Cambridge, England (D. Arnone, C. Ciesla, and M. Pepper, “Terahertz imaging comes into view,” Phys. World, pp. 35-40, April 2000).
In situ measurements of the transmitted or reflected terahertz energy incident upon a small sample are processed to reveal spectral content (broad signatures only), time of flight data (refractive index determination, amplitude and phase, and sample thickness), and direct signal strength imaging. The principle involves generating and then detecting terahertz electromagnetic transients that are produced in a photoconductive switch (PCS) or an optical crystal by intense femtosecond optical laser pulses. The laser pulses are beam split and synchronized through a scanning optical delay line and made to strike the terahertz generator and detector in known phase coherence. By scanning the delay line and simultaneously gating or sampling the terahertz signals incident on the detector, a time-dependent waveform proportional to the terahertz field amplitude is produced. Fourier transformation of this waveform yields information about the frequency spectral content. Transverse scanning of either the terahertz generator or the sample itself allows a 2-D image to be built up over time.
Other developments in terahertz spectroscopy include rapid scanning (S. Hunsche and M. C. Nuss, “Terahertz ‘T-ray’ tomography,” in Proc. SPIE Int. Millimeter SubmillimeterWaves Applicat. IV Conf, San Diego, Calif., July 1998, pp. 426-433.) and true 2-D sampling using charge-coupled device (CCD) arrays (Z. Jiang and X.-C. Zhang, “Terahertz imaging via electrooptic effect,” IEEE Trans. Microwave Theory Tech., vol. 47, pp. 2644-2650, December 1999.). In the Picometrix and Lucent Technologies systems, the generator and detector are based on the photoconductive effect in low-temperature-grown (LTG) gallium-arsenide (GaAs) compound semiconductor material, or radiation-damaged silicon on sapphire semiconductor. The Teraview system uses terahertz generation by difference-frequency mixing in a nonlinear crystal (ZnTe) and detection via the electrooptical Pockels effect (measuring the change in birefringence of ZnTe induced by terahertz fields in the presence of an optical pulse) as first demonstrated by Zhang at the Rensselaer Polytechnic Institute (RPI), Troy, N.Y. (see Q. Wu, T. D. Hewitt, and X.-C. Zhang, “Two-dimensional electro-optic imaging of THz beams,” Appl. Phys. Lett., vol. 69, no. 8, pp. 1026-1028, Aug. 19, 1996.). The femtosecond optical pulses are currently derived from relatively expensive Ti:Sapphire lasers, but other proposals include longer wavelength, especially 1.5 micron, solid-state systems that can take better advantage of fiber technology (see D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, “T-ray imaging,” IEEE J. Select. Topics Quantum Electron., vol. 2, pp. 679-692, September 1996). The RF signals produced by the optical pulses typically peak in the 0.5-2 THz range and have average power levels in the microwatt range and peak energies around a femtojoule. This makes T-ray imaging an attractive tool for medical applications (noninvasive sampling), as well as for nondestructive analysis of biological materials or electronic parts. One drawback of prior art designs is the need to scan the delay line slowly and over a distance of the desired wavelength resolution (e.g., a 1 GHz resolution would require a 7.5 cm scan of the movable optical delay line) and the inability to interrogate discrete frequencies of interest. The high degree of positional tolerance required to be maintained on the movable optical delay assembly limits the utility of this approach in applications where compact size and operation in uncontrolled environments are required with wide temperature excursions and/or shock and vibration. Also, in many cases, higher frequency resolution and accuracy are desired than is easily possible with scanning delay-line systems, such as in analysis of Doppler-limited molecular rotational transitions in low-pressure gases.
The need for a multi-octave tunable spectrometer in the THz region is justified by the new suite of applications relating to materials identification facing researchers and system developers today. Historically, the THz field has been dominated by radio astronomers and chemists usually concerned with detecting trace amounts of small gaseous molecules in the interstellar medium or in the Earth's upper atmosphere. The low pressure of the media involved would often lead to narrow, Doppler-limited absorption lines, sometimes less than 1 MHz in linewidth. In roughly the last decade, the THz application landscape has changed dramatically with the discovery and demand for detection and imaging of larger molecules, particularly biomolecules and bioparticles. This includes, for example, proteins and vitamins using frequency sweeps typically above 1 THz, and bacterial spores and nucleic acids using frequency sweeps typically below 1 THz. In most cases the biomolecular and bioparticle absorption occurs not in the form of narrow lines, but rather as broad “signatures”, typically 1 to 10 GHz or wider. Solid materials such as explosive agents and their precursors are also of particular interest for terahertz detection applications. Nano-structured materials are also of interest for high-resolution THz studies, due to the similarity in size of the nanostructures and the wavelength of THz radiation. Solid disordered materials typically have similarly broad absorption features due to phonons. Crystalline materials of interest may also exhibit sharper resonances. In many cases, there may only be a few limited frequency bands of interest that show strong THz absorption in a particular material of interest. A multi-octave spectrometer capable of measuring small discrete windows of frequencies with high resolution would allow faster measurement of signatures in the same session, increasing confidence and specificity.
In addition to the time-domain spectrometers noted above, frequency domain systems are also known (See the paper by Verghese et al., “Generation and detection of coherent terahertz waves using two photomixers,” Appl. Phys. Lett., vol. 73, no. 26, pp. 3824-3826, Dec. 28, 1998.). One prior art terahertz spectrometer system is described in U.S. Pat. No. 7,291,835, assigned to the common assignee, and hereby incorporated by reference. The system includes a laser illumination arrangement that generates a pair of source laser beams incident on a source photomixer device or PCS to cause emission of subcentimeter radiation, at least a portion of which interacts with the remote sample to generate a “sample influenced radiation” which is then incident on a detector photomixer device. A second pair of laser beams is incident on the detector to produce an optical component of the detector photocurrent that is offset in frequency with respect to the detected source laser energy. As a result, the detector generates a frequency down-converted electrical output signal responsive to and characteristic of the sample-influenced radiation.
The concept of photomixing is known from—for example—U.S. Pat. 6,348,683 which describes a method of generating quasi-optical signals using an optical-heterodyne converter or photometer source. Photomixer sources are compact solid-state sources that use two single frequency tunable lasers, such as diode lasers, to generate a terahertz difference frequency by photoconductive mixing in a photoconductive material. Photomixer sources using low-temperature-grown (LTG) GaAs have been used to generate coherent radiation at frequencies up to 5 THz. In particular the patent describes a transceiver for transmitting and receiving terahertz radiation. The transceiver includes a first laser that generates radiation at a first frequency and a second laser that generates radiation at a second frequency. The difference frequency, equal to the difference between the first and the second laser frequencies, is tunable by the user from microwave through terahertz frequencies by adjusting the frequency of one or both lasers. A transmitter includes a first photomixer that is optically coupled to the first and the second light source. A first radiative element or antenna is electrically coupled to the first photomixer. In operation, the first antenna radiates a terahertz signal generated by the first photomixer at the difference frequency. A receiver includes a second antenna to receive the signal from the target radiated by the first antenna. The second antenna generates a time varying voltage proportional to the terahertz return signal. A second photomixer is electrically coupled to the second antenna and is optically coupled to the first and the second light source. The second photomixer generates a current signal in response to the time varying voltage generated by the second antenna.
Commercial terahertz frequency-domain spectrometers, such as the Emcore PB7100 manufactured by Emcore Corporation in Alhambra, California, rely on coherent detection of the transmitted THz signal using a photomixer that is excited optically by a sample of the same heterodyne signal that produces the source THz signal. These frequency domain systems typically exhibit a characteristic repeating interference pattern in their raw data sets (i.e., the spectrogram collected by the instrument has a periodic pattern of peaks and nulls when plotted against frequency). This interference pattern is expected, as it is an inherent feature of any coherent unbalanced interferometer in which the input frequency is swept over a large range of frequencies. The peaks and nulls occur at frequency separation equal to the product of c/n L, where c is the speed of light, n is the index of refraction, and L is the path length imbalance between the interferometer arms. The interference could be avoided in principle if the path lengths of both the optical and THz paths were precisely balanced at all optical and THz frequencies. However, this is neither practical nor possible in many cases. Indeed, the propagation of the THz energy through air at sea level may introduce frequency dispersion due to the index of refraction of air, i.e., the index n becomes a function of frequency, so that n =n(f). Such a frequency-dependent index could cause the interference effect, even if the paths were perfectly balanced. Moreover, this frequency dependence could change with atmospheric conditions, such as dust and humidity. In the case of a sample being interrogated by the THz beam, the interferogram contains the detailed information of the complex index of refraction and absorption of the sample.
In the Emcore PB7100, this interference pattern may be “smoothed” by performing a running average of the data set, yielding a lower-resolution absorption spectrum for the sample being interrogated that provides useful information for absorption features with multi-GHz widths. Although such a technique mitigates the interference, the processing limits the frequency resolution of the data collected by the spectrometer, as well as throws away valuable information related to the phase of the signal at each frequency. Attempts to remove the interference and preserve frequency resolution, through mathematical processing of the data, have been unsuccessful to date for reasons that are not well-understood. It may be due to frequency dispersion of the THz signal in air, or due to frequency dispersion of the antenna elements, or a combination of the two.